When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Carotid artery atherosclerosis is a main cause of stroke and a major source of mortality and morbidity. Despite the relatively high prevalence of atherosclerosis at the carotid bifurcation, it remains difficult to predict which patients will experience devastating stroke. In the quest to identify carotid atherosclerotic lesions at risk for producing stroke, magnetic resonance imaging has found increasing utilization due to its non-invasive nature, excellent soft-tissue contrast, and ability to characterize carotid plaque structure and composition.
Numerous MRI protocols have been designed to image the structure and composition of the carotid arterial wall. A property of many of these protocols is the suppression of the arterial blood pool, which provides for clear delineation of the arterial wall from the lumen. Suppression of the blood pool is accomplished through application of specialized magnetization preparations including regional saturation bands, inversion-recovery based methods, and motion spoiling gradients. These magnetization preparations, however, each suffer from limitations that reduce the clinical utility. For example, some use quiescent inversion delay times to suppress the blood pool, are best suited for thin and non-contiguous section imaging, or suppress the blood pool for only a brief period of time.
Given these constraints and the general requirement of high spatial resolution imaging to adequately define the arterial wall, reported dark blood arterial wall MR protocols have largely been limited to static imaging protocols that, by definition and design, lack temporal resolution. These static imaging protocols, which may be in either two or three spatial dimensions, do not have the ability to display vascular pulsation occurring over the cardiac cycle, which limits the clinical usefulness of the information collected in the imaging process.
Recently, it was reported in Mendes J, Parker D L, Hulet J, Treiman G S, Kim S E. CINE turbo spin echo imaging. Magn Reson Med 2011; 66(5):1286-1292 that cardiac phase-resolved dark blood display of the carotid arteries could be achieved using a multi-slice 2D fast spin-echo protocol. However, this imaging technique provides only a limited extent of vascular coverage, cannot provide contiguous data with fine isotropic spatial resolution, and has not been validated against other measures of arterial distension.
Clinicians believe that the ability to precisely visualize and quantify motion carotid atherosclerosis throughout the cardiac cycle may enable better characterization of the disease and prognosticate future cerebrovascular events. Unfortunately, there is no MRI method that provides large coverage dark-blood display of the carotid arteries with submillimeter isotropic spatial resolution over the cardiac cycle.
Therefore, it would be desirable to have a system and method for acquiring a plurality of 3D, time-resolved image sets over the cardiac cycle in a clinical reasonable scan time and without signal from blood within the vascular or cardiac structures obscuring clinically useful information.